What disease are you trying to control? You might want to bring a sample to the Cape Cod Extension Master Gardener office to have the plant diagnosed and the proper control given.
We generally do not make pesticide recommendations without seeing the problem and sometimes, culturing the plant for a specific disease. You can also call the UConn Home & Garden Education Center (877) 486-6271 with more details as to what disease symptoms you are seeing and they can also make control suggestions.
Hope this helps.